In metallic conductor communicative transmission systems, a nagging problem has been electromagnetic interference (EMI). A metallic transmission medium between a transmitter and a receiver effectively is an antenna. As such, any signal on a pair of metallic conductors which connect the transmitter to the receiver may both radiate EMI and pick up EMI from other electrical equipment.
EMI pickup currents will be delivered to the receiver where they will interfere with the received signal. In a coaxial cable which comprises a solid, centrally disposed inner conductor and an outer tubular conductor separated from the inner conductor by a dielectric material, the outer conductor is effectively a shield which keeps currents inside and which also prevents the pickup of spurious external currents.
Radiated or outwardly directed EMI, that is, EMI outgoing from a transmission line, is undesired because of its adverse effects on reception by neighboring equipment. The Federal Communications Commission (FCC) has set limitations on the maximum signal that can be radiated. At high frequencies, particularly at those such as 16 megabits, for example, which are used in data transmission, the higher harmonics are most apt to cause outwardly directed EMI. Accordingly, it long has been sought to find arrangements for effectively combating EMI.
A first step toward combating EMI is balanced mode transmission. A pure balanced mode signal is one in which at any instant the voltage with respect to ground on one conductor of a pair is equal and of opposite polarity to the voltage of the other conductor of the pair. The balanced mode is sometimes referred to as the differential mode. On the other hand, a pure longitudinal mode signal is one in which at any instant the voltage with respect to ground on one conductor of a pair is equal to the voltage on the other conductor. A typical signal comprises a balanced mode component and a longitudinal mode component.
Traditional balanced mode systems include a Balun or transformer in the circuit at the transmitting end and at the receiving end. Transmitting in the balanced mode is acceptable except that the practical limit of balance is about 30 dB, which is to say, there remains an accidental longitudinal source. At the receiving end there is an analogous occurrence. EMI pickup tends to be in the longitudinal mode, i.e. there are equal voltages on the two conductors of a pair. The Balun or transformer at the receiving end responds to the balanced signal and cancels out the longitudinal EMI voltages, but once again the practical limit of cancellation is about 30 dB.
Another approach to solving the problem of EMI has been to provide a metallic shield around each conductor pair individually or around a plurality of pairs of twisted insulated metallic conductors. This approach is similar to the use of coaxial cable in which the outer tubular conductor performs effectively as a shield. However, there are drawbacks to the use of a shield in cable construction. The shield itself and its formation about the conductors in tubular form is expensive, increases the bulk of the cable and complicates connections. Further, it increases the attenuation of the signal to be delivered.
Still another solution has been the inclusion of a longitudinal choke in the transmission circuit. The longitudinal choke includes two windings, one for each conductor of a pair. A longitudinal choke is inductive to currents flowing in the same direction on the two conductors. Such currents are called longitudinal currents. The two windings are in parallel. If one winding is reversed, the resulting device would be a load coil. A load coil is inductive to currents flowing in opposite directions on the two conductors. Such currents are called balanced or differential currents. The inclusion of a longitudinal choke is effective to cause a substantial reduction in the longitudinal current which flows.
Longitudinal chokes have been used in cables in which the twisted pairs of insulated conductors are unshielded. The prior art also includes the use of a longitudinal choke in combination with a shielded pair. Tests have shown that the degree of EMI suppression resulting by using an unshielded pair with a choke is not as good as with a shielded pair which includes a longitudinal choke.
Notwithstanding the improvement in EMI suppression which is achieved by using a longitudinal choke with a shield, it has been desired to eliminate the use of the shield for the reasons set forth hereinbefore. That desire, seemingly, has not been satisfied by the prior art.
Seemingly, the solutions of the prior art to the problem of providing a local area network cable which can be used to transmit data bits without either radiating or picking up EMI have not yet been totally satisfying. What is needed and what is not yet available is a transmission system which includes unshielded transmission media and facilities for suppressing substantially EMI, which are compatible with balanced mode transmission equipment and which can be readily installed, fit easily into building architectures, and are safe and durable. The sought after arrangement should be one that may be used with a well known D-inside wiring which comprises a plurality of non-shielded twisted insulated conductor pairs enclosed in a plastic jacket.